 

GIS Map Gallery:
Earthquake Probability Map


Natural hazards, such as earthquakes, are important to corporate
and industrial site selection. Not only could those hazards destroy properties but they
can significantly disturb the operations of company headquarters or plants. Express
couriers may not be able to reach the facility, telecommunications may be greatly reduced,
employees may try to recover and not be able to go to work...
Sources: 
Applied Technology Council
National Geophysical Data Center 
Map Basis: 
Based on acceleration of gravity and
epicenters of actual significant events showing magnitude of event (Richter scale) 
Date: 
1900  1994 
Data Scale: 
Data aggregated to county level 

Seismic
risk maps for the United States are based on either relative hazard or probability.
The probabilistic approach to seismic risk, while more complicated, is currently the
preferable method^{1}. This Earthquake Probability Map uses seismic
coefficient as an indicator for earthquake probability. It displays the counties
where maximum values of horizontal ground acceleration caused by seismic shaking that,
with a 90% probability, are not likely to be exceeded in 50 years. The darkest red
areas represent the greatest seismic hazard, interpreted as an area where the horizontal
acceleration from seismic shaking of greater than four meters per second per second (40%
of the acceleration of gravity) can be expected, with odds of only one in ten to be
exceeded in a 50 year period.
The seismic data is distributed in five classes using the natural
break methodology. This method finds grouping and patterns inherent in the data
through the identification of breakpoints between classes using a statistical formula
(Jenk's optimization) that minimizes the sum of the variance within each of the classes.
Natural Breaks finds grouping and patterns inherent in the data.
For additional information, the Earthquake Probability Map was
overlaid with the location of past significant earthquake epicenters from the National
Geophysical Data Center. The original seismic coefficient data used to produce this
map originated from the Applied Technology Council (ATC), a nonprofit corporation set up
by the Structural Engineers Association of California. 
^{1} Keller, Edward A., 1989, Environmental Geology, p.121
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